The invention concerns a process for regeneration of electrolytes, in particular of Na2SO4 from spent pickling liquid, or pickle liquors, for stainless steel, in particular, stainless steel strips.
The xe2x80x9cneolytexe2x80x9d process using a neutral electrolyte has become widely accepted all over the world for electrolytically pickling stainless steel strip, in particular cold strip. This process provides for the strip to be de-scaled in a sodium sulphate solution by means of electric current. In the process, metals build up in the electrolytexe2x80x94i.e., the sodium sulphate solution in this case. Chromium is dissolved in the form of chromate (Cr6+), while the other metals such as iron and nickel are present in the form of hydroxides.
The metal content of the pickling acid is kept constant by continuous extraction of electrolyte from the pickling bath and its substitution with fresh sodium sulphate solution. This causes considerable operating expenses for sodium sulphate and for the disposal of the spent electrolyte solution.
Briefly stated, the invention in a preferred form is a process for regenerating the electrolyte present in the pickle liquors which reduces both the operating cost and the environmental impact of so-called neolyte pickling plants.
In the process according to the invention, this objective is achieved by adding NaxHySzOv in an acid solution, which causes Cr6+ to be reduced to Cr3+, with the suffixes being x=0 to 2, y=0 to 2, z=1 to 6, v=2 to 6. By adding this substance, Na2SO4 is produced in any of the reactions and can be easily recycled to the process.
An advantageous design of the invention is characterized by the fact that no foreign atoms are introduced to the solution. This avoids formation of other, undesirable reaction products apart from Na2SO4.
An advancement of the invention is characterized by the fact that the reaction takes place at a pH in the range of approx. 2-3. The decrease of the pH is favorably effected with H2SO4, which also means that no foreign atoms are introduced to the solution. Having the reduction take place in the acid range speeds up the reaction in an advantageous manner.
An advantageous design of the invention is characterized by the fact that the acid solution produced is neutralized by adding NaOH. This neutralization with NaOH causes Na2SO4, which can be easily recycled to the process, to be formed also in this step.
An advantageous advancement of the invention is characterized by the fact that the metal hydroxides produced by neutralization are separated from the pickling acid (Na2SO4) and the solution is recycled to the pickling process. By recycling the pickling acid, the high cost of fresh pickling acid is avoided, and by separation of the metal hydroxides from the pickling acid these can be deposited at a higher concentration, i.e., with a lower content of residual acid.